Circuits that employ an automatic gain control for controlling the gain of a photomultiplier tube (PMT) or similar photodetector are known in the prior art.
For example, the prior art circuit shown in FIG. 1 employs an operational amplifier (op-amp) 10 in a integrating configuration, where resistor R and capacitor C set the integration time constant as the product R*C seconds. The filtered DC portion of the PMT output signal 12 (hereafter referred to as “PMT output”) is compared to a pre-selected DC reference 14. The output of the op-amp 10 is applied to the bias control 16 of the PMT, thereby to drive the PMT at a level such that the PMT output matches the DC reference. For example, if the level of the PMT output reaching the op-amp 10 is below the DC reference 14, the op-amp will provide to the PMT bias control 16 a signal for driving the PMT at a relatively higher level until the PMT output matches the DC reference.
A controller (not shown) monitors and conditions the signals directed to and from the op-amp 10. For example, in instances where the DC reference is user-selected, the system provides a user interface for receiving the selected input from the user, which input the controller converts as necessary to a corresponding DC reference level applied to the op-amp 10.
It is important that, despite the selected DC reference, the PMT gain be limited to an amount that does not cause the applied bias voltage to overdrive, hence damage, the PMT. Moreover, such techniques for limiting the PMT gain should not interfere with the precision with which the PMT output may be established (by selection of the DC reference) below that limited level.